In the field of electric motors and motor drives, a motor is connected to a motor drive, which provides electrical power to the motor in a controlled fashion, wherein the motor drive may take a variety of forms, such as a current source inverter (CSI), voltage source inverter (VSI) or the like. Such motor drives are commonly employed to provide electrical power to motors. The motor drive may be employed in order to provide speed control, torque control, and/or to control other motor performance characteristics. For AC motors, electrical power is converted in the motor drive from supply power, typically AC power from a utility or other source, into DC. The DC power is then converted, for example using an inverter stage, into AC power at a controlled frequency and amplitude, which is provided to the motor windings.
The AC to DC converter stage in the motor drive, and/or the subsequent inverter stage, often comprise solid state semiconductor-based switching elements, such as silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate commutated thyristors (GCTs), insulated gate bipolar transistors (IGBTs), or other switching devices. As the switching elements are activated in the AC to DC converter stage or the inverter stage, common-mode voltages are produced. This common-mode voltage appears in the output phases of the motor drive, and hence, on the motor windings. Where a neutral associated with the input power source is grounded, this common-mode voltage appears between the motor windings and ground, and may reach levels beyond the motor winding insulation ratings. Consequently, such common-mode voltages may damage the motor or decrease the life expectancy thereof.
Conventional approaches to addressing this motor drive common-mode voltage problem have included designing the motor winding insulation to higher voltage ratings. However, this approach provides no solution for existing motors, which do not include insulation capable of withstanding the common-mode voltage levels. Another approach has been to provide an isolation transformer between the power source and the motor drive AC power input terminals. However, such an isolation transformer must be rated to handle the common-mode voltage levels. In addition, isolation transformers add cost and occupy space in the motor drive system. Thus, there is a need for apparatus and techniques to reduce or minimize the common-mode voltages in a motor drive, which do not require isolation transformers, and which operate in association with existing motors.